1. Field of the Invention
The present invention relates generally to a universal joint and, more particularly, to a universal joint incorporated into a steering apparatus of, e.g., an automobile and utilized for transmitting motions of a steering shaft to a steering gear.
2. Related Background Art
For example, Japanese Utility Model Laid-Open Publication No. 59-56424 discloses a structure as shown in FIGS. 13-14, of a universal joint, incorporated into a steering apparatus, etc. of an automobile, for transmitting rotational motion between a pair of rotary shafts which do not exist on the same straight line. This universal joint 1 includes a pair of yokes 2a, 2b, a single piece of cross or cruciform shaft 3 for connecting the pair of yokes 2a, 2b to each other so as to be capable of oscillating and displacing, and bolts which are, though not illustrated, used for fixing the respective yokes 2a, 2b to end portions of unillustrated shafts.
Proximal ends of the yokes 2a, 2b, which are manufactured by effecting press working upon a metal plate such as a steel plate exhibiting a sufficient rigidity, are respectively provided with partially cylindrical joint portions (herein after joint cylindrical portions) 4a, 4b. Each of those joint cylindrical portions 4a, 4b has a discontinuous portion 5 at one place in a circumferential direction, an inside diameter of which is elastically reducible. A rugged portion like serration grooves is formed along an inner peripheral surface of each joint cylindrical portions 4a, 4b. Further, first and second flange portions 6, 7 are, with the above discontinuous portion 5 sandwiched therebetween, provided in parallel with each other in given portions of the respective joint cylindrical portions 4a, 4b. The first and second flange portions 6, 7 are formed with through-holes 8, 9 coaxial with each other in such a direction as to bear a cross positional relationship with each of the joint cylindrical portions 4a, 4b. Moreover, a nut 10 is fixed by welding to an outer surface (a lower surface in FIGS. 13 and 14) of the second flange portion 7, and a screw hole formed in this nut 10 is disposed coaxiallly with the through-holes 8, 9. Further, a pair of joint arm members 12, 12 extend in an axial direction from two positions, opposite to each other in a diametrical direction, of one edges of the respective joint cylindrical portions 4a, 4b in the axial direction. Then, circular holes 13, 13 are formed coaxially with each other in tip portions of the joint arm members 12, 12.
Further, the cross shaft 3 has four lengths of shaft members 14, 14. These shaft members 14, 14 are provided in such a state that the shaft members 14, 14 adjacent to each other are orthogonal to each other. These shaft members 14, 14 are rotatably supported through bearings inwardly of the circular holes 13, 13 formed at the tip portions of the joint arm members 12, 12. More specifically, bearing cups 15, 15 formed in a bottomed cylindrical shape by use of a rigid metal plate such as a case hardening steel, are press-fitted and fixed inwardly of those circular holes 13, 13. The tip portions of the shaft members 14, 14 are inserted in those bearing cups 15, 15, and a plurality of needles 16, 16 are provided between inner peripheral surfaces of the bearing cups 15, 15 and outer peripheral surfaces of the tip portions of the shaft members 14, 14, thus constituting a radial needle bearing.
The thus constructed universal joint 1 has the pair of yokes 2a, 2b fixed to end portions of a pair of shafts each disposed on a non-identical straight line, and a rotating force is transmittable between the pair of shafts. In order to fixedly join the yokes 2a, 2b to the end portions of the respective shafts, a bolt 17 (see FIGS. 1-11 which will be described later penetrating the through-holes 8, 9 formed in the first and second flange portions 6, 7, is screwed into the screw hole of the nut 10 and then fastened. That is to say, the bar member 18 of the bolt 17 is inserted into the two through-holes 8, 9 from the opposite side (an upper side in FIGS. 13 and 14) to the screw hole 11, and a male screw portion 19 provided at a proximal end of the bar member 18 is screwed into the screw hole 11. At the same time, a one-side surface of the head member 20 formed at the proximal end of the bar member 18 is brought into contact with a one-side surface (an upper surface in FIGS. 13 and 14) of the first flange portion 6. Bolts 17 are fastened from this corresponding state, and inside diameters of the joint cylindrical portions 4a, 4b are thereby decreased, whereby the outer peripheral surfaces of the ends of the above shafts are pressed by the inner peripheral surfaces of the joint cylindrical portions 4a, 4b. The outer peripheral surfaces of the ends of the shafts are also formed with serration rugged portions, and therefore the yokes 2a, 2b are firmly fixedly joined to the end portions of the shafts by fastening of bolts 17.
Note that the formation of the screw hole 11 through which the male screw portion 19 of the bolt 17 is screwed to the second flange portion 7 does not necessarily involve fixing the nut 10 to the outer surface of the second flange portion 7 by welding as illustrated in FIGS. 13 and 14. In addition, as disclosed in, e.g., Japanese Patent Laid-Open Publication No. 58-193935, a cylindrical nut member is fixedly fitted inside to a through-hole 9 formed in a second flange portion. Alternatively, as disclosed in Japanese Patent Laid-Open Publication No. 3-41220 and Japanese Utility Model Laid-Open Publication No. 55-68726 that will hereinafter be described, a screw hole is formed directly in a second flange portion 7 in place of the through-hole 9.
A distance between the first and second flange portions 6, 7 is narrowed by fastening the bolt 17. If a particular measure is not taken, however, the flange portions 6, 7 become unparallel to each other so that a distance between the tip portions (left end portions in FIG. 14) of the respective flange portions 6, 7 is especially reduced, whereas a distance between proximal ends (side edge portions of the joint cylindrical portions 4a, 4b, and right end portions in FIG. 14) thereof becomes larger than between the tip portions. As a result, a force in a bending direction is applied to the bolt 17 spanning the two flange portions 6, 7, and there might be a possibility in which the bolt 17 is broken off in an extreme case such as fastening the bolt 17 with a torque largely exceeding a specified value. Such being the case, according to the structure illustrated in FIGS. 13 and 14, the tip portions of the two flange portions 6, 7 are formed with bent portions 21, 21 bent toward the face-to-face flange portions 6, 7. When the distance between the tip portions of the two flange portions 6, 7 is reduced by fastening the bolt 17, tip edges of the two bent portions 21, 21 contact each other, thereby preventing the distance between the tip portions of the two flange portions 6, 7 from being further shrunk. Accordingly, the bolt 17 is prevented from being deformed enough to cause a damage thereto by decreasing a degree to which those two flange portions 6, 7 become unparallel.
Note that Japanese Utility Model Laid-Open Publication No. 59-56424 discloses a structure for decreasing the degree to which the pair of flange portions with a distance reduced by fastening the bolt become unparallel, and, in addition, the known structures are dealt with in Japanese Patent Laid-Open Publication No. 3-41220 and Japanese Utility Model Laid-Open Publication No. 55-68726. According to the structures disclosed in those Publications, some portions, for forming the first and second flange portions 6, 7, of the metal plate are folded double at 180 degrees or formed in padding to increase a thickness.
According to the prior art structures, the degree to which the first and second flange portions 6, 7 become unparallel is decreased for preventing the bolt 17 from being broken off by fastening the bolt 17. It is troublesome to manufacture those first and second flange portions 6, 7, and further it is inevitable that the weight of the first and second flange portions 6, 7 increases.